The present invention relates to coated granular magnetite particles and a process for producing the same. More particularly, the present invention relates to granular magnetite particles coated with M.sub.x Fe.sub.2+y O.sub.z (wherein M is Zn or Co, 0.4.ltoreq.x.ltoreq.1, x+y=1, and 4.ltoreq.z.ltoreq.4.3), which have an excellent heat resistance and a high tinting strength, and a process for producing the same.
Coated granular magnetite particles according to the present invention are chiefly intended as materials for coloring pigments for coating materials, printing inks and resins, and materials for magnetic toners and magnetic carriers.
Since granular magnetite particles are black, they are widely used as materials for a black coloring pigment which is dispersed in a vehicle or mixed with a resin.
In addition, since granular magnetite particles are black ferrimagnetic particles, they are mixed with and dispersed in a resin so as to produce composite particles which are also widely used as a particle material for magnetic toners and magnetic carriers for electrostatic copying.
When granular magnetite particles are used as a material for a black coloring pigment, if the granular magnetite particles have too high a coercive force, the magnetic reagglomeration is produced among particles, which makes the dispersion of the particles in a vehicle or a resin difficult. From the point of view of dispersibility, granular magnetite particles are required to have a coercive force of not more than about 300 Oe.
When granular magnetite particles are used as a material for a magnetic toner, (1) it is preferable that the granular magnetite particles have a high coercive force in order to improve the carrying property and the fluidity of the magnetic toner, but in order to improve the dispersibility of the magnetic particles and the turbulence of the magnetic toner, to activate the contact, separation and the friction between the magnetic particles and between the magnetic particles and the sleeve of a developing equipment, and to improve the developing property, the coercive force thereof must be not more than about 300 Oe.
(2) On the other hand, when used as a material for a magnetic toner, in order to prevent the reduction in the image density in continuous copying and to ameliorate defective development such as a part of image missing, the coercive force of the granular magnetite particles is preferably as low as possible, particularly not more than 150 Oe.
As described above, granular magnetite particles are widely used as materials for a black coloring pigment. Since they are often exposed to a high temperature of not lower than 150.degree. C., particularly not lower than 200.degree. C., they are required to keep a stable tone at a temperature of not lower than 150.degree. C., particularly not lower than 200.degree. C. (this property is referred to as "heat resistance" hereinunder).
This fact is described in Japanese Patent Publication (KOKOKU) No. 54-7292 (1979) as " . . . When the pigment is used as a coloring agent for a thermoplastic resin such as polyethylene, polypropylene, polystyrene and ABS, since these thermoplastic resins are formed and processed at a temperature as high as approximately more than 200.degree. C., especially a yellow iron oxide pigment is not immune to discoloring, so that the uses thereof are limited. . . . " and in Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 (1980) as "Such magnetic powder for a one component magnetic toner is generally required to have the following properties. . . . iv) To have a blackness which stands practical use. Although it is possible to add a coloring agent to the magnetic toner, it is preferable that the magnetic powder itself has a black color and that no coloring agent is used. v) To have a high heat resistance. It is necessary that the tone, particularly the blackness and the electromagnetic characteristics are sufficiently stable in a temperature range of 0.degree. to about 150.degree. C. . . . "
The blackness of granular magnetite particles depends upon the Fe.sup.2+ content and is apt to become higher with the increase in the Fe.sup.2+ content. The phenomenon of the tone of the granular magnetite particles changing from black to brown is caused by the oxidation of Fe.sup.2+ in the granular magnetite particles into Fe.sup.3+, thereby transforming the granular magnetite particles into maghemite particles when the granular magnetite particles are exposed to a temperature as high as 150.degree. C., especially to a temperature of about 200.degree. C.
Furthermore, since if it is possible to color with a small amount of granular magnetite particles, it is advantageous in resources-saving and energy-saving as well as the operability such as handling property, granular magnetite particles are required to maintain the blackness before heating as much as possible so as to have a high tinting strength.
Granular magnetite particles are conventionally produced by what is called a wet-process which comprises the steps of: blowing an oxygen-containing gas into a suspension containing Fe(OH).sub.2 colloids or Fe-containing precipitates which are obtained by adding an aqueous ferrous salt solution to an aqueous alkali solution such as an aqueous alkali hydroxide solution and an aqueous alkali carbonate solution.
Various attempts have been made to improve the properties of granular magnetite particles. For example, there are known a method of coating granular magnetite particles with an insoluble inorganic compound such as zinc hydroxide, zinc phosphate, zinc phosphite, aluminum phosphate and silica (U.S. Pat. No. 4,082,905), a method of coating granular magnetite particles with a ferrite containing 1.5 to 13 mol % of a divalent metal other than Fe (Japanese Patent Application Laid-Open (KOKAI) No. 3-67265 (1991)), and a method of coprecipitating an oxide of Zn, Mn, Ni, Co, Mg, Cu or Cd at the time of producing granular magnetite particles (Japanese Patent Publication (KOKOKU) No. 3-48505 (1991)).
More specifically, U.S. Pat. No. 4,082,905 discloses an Fe.sub.3 O.sub.4 iron oxide black pigment having incorporated therein up to about 50 mole percent of .gamma.-Fe2O.sub.3 plus MeFe.sub.2 O.sub.4 (wherein Me is at least one of magnesium, manganese, cobalt, nickel, aluminum, chromium, titanium, copper, zinc and cadmium) having a specific surface according to BET of greater than about 12 m.sup.2 /g, up to about 10% by weight of a substantially insoluble inorganic compound as coating and a stability to oxidation with air under standardized conditions of greater than about 130.degree. C.
Japanese Patent Application Laid-Open (KOKAI) No. 3-67265 (1991) discloses a magnetic toner for developing a electrostatic image, comprising at least a binder resin and a spherical magnetic powder containing spherical magnetic particles, the spherical magnetic particles having a surface layer composed of a different composition from the core portion, and the surface layer being composed of a ferrite containing 1.5 to 13 mol % (calculated as divalent metal ions) of an oxide of a divalent metal other than iron.
Japanese Patent Publication No. 3-48505 (1991) discloses a magnetic toner for developing an electrostatic latent image, comprising a binder resin and a magnetic powder as the main ingredients, the magnetic powder being produced by a wet-process and containing an oxide of Zn, Mn, Ni, Co, Mg, Cu or Cd coprecipitated in the process of production.
As one of improving methods of various properties of the magnetic particles, "an acicular ferromagnetic iron oxide particles for a magnetic recording medium, comprising Co-coated .gamma.-Fe.sub.2 O.sub.3 particles as a core and an iron oxide layer containing zinc and a ferrous formed on the surfaces of Co-coated .gamma.-Fe.sub.2 O.sub.3 particles" is known in the field of a magnetic particles for magnetic recording, as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-165703 (1985). However, a method of coating magnetite particles with zinc and/or cobalt ferrite has not yet been proposed in the field of a pigment.
Granular magnetite particles having as low a coercive force as possible, particularly a coercive force of not more than 300 Oe and yet having high heat resistance and tinting strength are in the strongest demand at present, but the granular magnetite particles obtained in any of the above-described methods have poor heat resistance and tinting strength, although they have a coercive force which is as low as 40 to 130 Oe.
These granular magnetite particles begin to transform into maghemite particles at a temperature of about 130.degree. C., and the color changes from black to liver brown, so that the tinting strength is low, as shown in the comparative examples which will be described later.
Although the particles described in U.S. Pat. No. 4,082,905 have an improved heat resistance, it cannot be said to be satisfactory.
In the particles described in Japanese Patent Application Laid-Open (KOKAI) No. 3-67265 (1991), the amount of charge can be controlled, but the heat resistance is not improved, and the particles begin to transform into maghemite particles at a temperature of about 140.degree. C., as shown in Comparative Example 4 which will be described later. That is, the particles have a poor heat resistance.
The granular magnetite particles described in Japanese Patent Publication Nos. 59-43408 (1984) and 3-48505 (1991) are not the particles with improved heat resistance and tinting strength which are aimed at by the present invention.
The above-described the coating of the acicular magnetic particles with zinc ferrite (Japanese Patent Application Laid-Open (KOKAI) No. 60-165703 (1985)) in the field of magnetic recording are to improve the saturation magnetization and the change of the saturation magnetization with passage of time in response to the demand for high-density recording, but the improvement of heat resistance and tinting strength which are aimed at by the present invention, is not the target at all. Actually, these particles have a low heat resistance and begin to transform into maghemite particles at a temperature of about 120.degree. to 145.degree. C., as shown in the comparative examples which will be described later.
As to the charging property of the particles, it is preferable that the magnetic particles are positively charged in the case of a positive charge toner or that the magnetic particles are negatively charged in the case of a negative charge toner, as described in Japanese Patent Application Laid-Open (KOKAI) No. 60-117259 (1985) as "On the other hand, it is necessary that the toner is adjusted so as to be positively or negatively charged depending on the kind of a photosensitive material. Magnetic particles such as magnetite particles have a property of negatively charging. When such magnetic particles are used for a magnetic toner provided with a positive charging property, the phenomenon that a positively charged portion and a negatively charged portion coexist on the surface of one toner particle or the charging properties are different in toner particles, may be produced due to the existence of the magnetic particles exposed on the surface of the toner. This phenomenon is supposed to be one cause of abnormality in the image copying properties under some circumstance in a one component developing system using a magnetic toner. It is, therefore, expected that the magnetic particles in the toner are positive charge particles."
It is also preferred that the amount of charge of the toner is adjusted in an appropriate range so as to fit a developing equipment. For this purpose, the amount of charge of the whole magnetic particles is adjusted to an adequate amount.
As a result of various studies undertaken by the present inventors so as to achieve the above-described problems in the related art, it has been found that by dispersing granular magnetic particles to be treated in an aqueous dispersion, adding and mixing 1.0 to 39 mol % (calculated as Fe.sup.2+ and based on the total amount of Fe in granular magnetite particles to be treated in the aqueous dispersion) of an aqueous ferrous salt solution, 0.5 to 6.0 mol % (calculated as M and based on the total amount of Fe in the granular magnetite particles to be treated) of an aqueous M salt solution and an aqueous alkali hydroxide to and with the obtained aqueous dispersion in a non-oxidizing atmosphere, thereby adjusting the concentration of the OH group in the dispersion to 0.3 to 1.0 mol/l, and blowing an oxygen-containing gas into the aqueous dispersion at a temperature of not lower than 50.degree. C., the obtained granular magnetite particles coated with M.sub.x Fe.sub.2+y O.sub.z (wherein M is Zn or Co, 0.4.ltoreq.x.ltoreq.1, x+y=1, and 4.0.ltoreq.z.ltoreq.4.3), have the change ratio (%) in the Fe.sup.2+ content of not more than 8% and the change in the hue of not more than 0.8 after heating the coated granular magnetite particles at a temperature of 200.degree. C. for 1 hour, and show black and a very excellent heat resistance. The present invention has been achieved on the basis of this finding.